There is a recognized need to locate mobile terminals. Mobile terminals include cell phones, smart phones, note computers, tablet computers such as the iPad, and other mobile computing devices wirelessly connected to a communication network. The location of a mobile terminal is needed for 911 calls, enabling of location-based services, mapping, navigation assistance, and other applications. Multiple technologies have been deployed for locating mobile terminals including the use of Global Positioning System (GPS) receivers in the mobile terminals, Assisted-GPS (AGPS) in which the GPS receiver is aided by data obtained from the communication network, Time Difference of Arrival (TDOA) in which differences in signal receipt times to multiple network communication sites are used to estimate mobile terminal locations, signal multipath maps, signal power maps, an other techniques. Of the many technologies deployed, the use Global Navigation Satellite System (GNSS) signals such as GPS is becoming increasing prevalent. Other GNSS systems include the Russian GLONASS system, the Chinese COMPASS system, and the European Galileo system.
GNSS systems are typically dual-use systems having a civilian component with publicly disclosed coding to support general use and a secure component with undisclosed and encrypted coding for military applications. For example, in the GPS system the C/A code is a publicly disclosed code that is modulated onto the 1575.42 MHz (L1) carrier of the GPS signal to support most commercial applications. By comparison, military receivers make use of the Y-code, a composite of a publicly-disclosed P-code and an undisclosed/encrypted W-code.
Since the C/A code is known, it is relatively simple to spoof a C/A code GPS receiver, whereby the receiver is fooled into producing an incorrect location or navigation fix by a spoofer artificially generating signals with C/A code modulation to present to the receiver. In contrast, it is very difficult to spoof a Y-code receiver since the W-code is not known outside of organizations authorized to make use of it. Without knowledge of the W-code it is virtually impossible to generate a-priori signals representing a false location to present to the receiver. As such, the W-code is one of the GPS's anti-spoof (AS) techniques. It precludes an adversary from deceiving the Y-code GPS receiver for obvious reasons in military applications.
As the ubiquity of mobile computing devices grows, it is becoming increasingly important to extend anti-spoofing capability to GNSS receivers in the civilian world. For example, the problem of falsified transactions, particularly financial transactions, has become an increasing burden on consumers, banks, stores, and the national financial infrastructure. One element in a system to defend against electronic fraud and identify theft is the capability to ensure that the location of a mobile terminal is known, even in the threat of a spoofer. Therefore, there is a need to verify that the location reported by a mobile terminal is true.